It consists of a transparent glass tube constructed in the form of an elongated U and partially filled with a liquid, most commonly water or mercury. One end of the tube is connected to the pressure tap, and the other end is connected to the other pressure tap (or it may be left open) When there is a pressure difference between the two ends of the tube, the liquid level goes down on one side of the tube, and up on the other side. The difference in liquid levels from one side to the other indicates the difference in pressure.